The Sussex-based agency has been tasked with devising a programme to raise awareness and understanding of peripheral neuropathic pain (PNP).
The condition is caused by damaged nerves in the peripheral nervous system - the nervous system outside the brain and spinal chord - and often flares up following another condition, such as a stroke, spinal cord injury or multiple sclerosis.
Health professionals say the disease is particularly difficult to treat because every case is different.
A key plank of the brief will be to promote treatment patches, Qutenza, which contain the active ingredient capsaicin which relieves nerve pain in the skin by desensitising nerve receptors.
The manufacturer is part of a Tokyo-based global firm which employs 4,200 staff around the world, and is the holder of a five year European Uunion marketing authorisation awarded in 2009.
Mindy Dooa, corporate comms director at Astellas Pharma Europe said: 'The team presented some fresh, creative ideas and strong strategic insights, which was combined with a good understanding of the stakeholder environment.'
Simon Hackett, deputy managing director at Pegasus said the programme was to focus on Qutenza as a 'topical treatment for patients living with this painful and difficult-to-treat condition. Working on such a significant European programme is a great opportunity.'
This is the first time Pegasus has worked with Astellas.